Cutting an archway in a wall of a model of a building
You could model the arch's shape from a plane and use Knife Project.




Select first the arch then the wall in Object Mode
Change to Edit Mode and apply Knife Project


After that you should have a model like:



If  you want to connect the arches select them by Shift-Alt-RMB and Bridge Edge Loops

Alternativley (as suggested by Gandalf3) you could use the Boolean Modifier.

For this technique you would need to extrude the arch like the red object here:




Align the objects 
Add a Boolean Modifier to your wall, and set operation to Difference
Apply the modifier




The result should look like: